Selective segregation of hydrocarbon constituents in a combination crude distillation process



May 3, 1960 R. P. CAHN 2, 35,462

SELECTIVE SEGREGATION OF HYDROCARBON CONSTITUENTS IN A COMBINATION CRUDEDISTILLATION PROCESS Filed Aug. 2, 1954 HH mtDwI Robert P Cahn InventorBy dX/K Attorney United States Patent SELECTIVE SEGREGATION orHYDROCARBON CONSTITUENTS m A COMBINATION CRUDE DISTILLATION rnocnssRobert P. Cahu, Elizabeth NJ assignor to Esso Research and EngineeringCompany, a corporation of Delaware Application August 2, 1954, SerialNo. 441,014

9 Claims. c1. 208-81) [This invention relates to the refining of mineraloil, and more particularly to an improved refining process involvingintegrated hydrocarbon cracking and fractionating steps, as well as toimproved apparatus in which said fractionating step is carried out.

In the past, it has been well known in the petroleum refining art toproduce topped or reduced crudes by subjecting the crude to variousdistillation operations. This topped crude, comprising gas oilconstituents boiling in the range of from about 6501100 F., and higherboiling constituents, is then handled in a manner to secure a maximumsegregation of these gas oil constituents. Normally this is secured bythe utilization of a vacuum distillation operation. The gas oil fractionsegregated in the vacuum distillation operation is then usually passedto a catalytic or thermal cracking process. Such prior art processeshave a number of disadvantages, among these being the necessity forproviding a plurality of product fractionators yielding several streamsof prodnets of desirable boiling ranges. For economical heat recovery,large numbers of heat exchange apparatus are required, both within eachunit and in combination between units. Vast tank facilities must beprovided to permit storage of the various products prior to blending indesired proportions. The vacuum distillation equipment used for workingup the reduced crude is expensive with respect to investment, operation,and maintenance. As a result of these complications, conventional typecombination processes must be operated on a relatively large scale to beeconomical. Normally, refining capaci ties in excess of about 20,000barrels per day of crude are required to make operations of this typepay, while smaller refineries must be ,designed on the basis of an oftenundesirably high output of heavy fuel oil and other products of arelatively low'commercial value.

It is an object of the present invention, therefore, to provide acombination crude distillation and cracking process wherein the vacuumstill for reduced crude distillation'is eliminated.

It is a further object of the present invention to provide a processpermitting substantial savings in fractionating equipment and tankagefacilities.

1 It is a still further object of the invention to provide a processwhereby heavy bottoms from reduced crude may be recovered substantiallyuncontaminated by gas oilfractions.

I It is a still further object of the invention to provide a processwhereby cracked gas oil may be recovered for recycling to a crackingunit. 7

- It is a still further object of the invention to provide aprocess-whereby a portion of the cracked gas oil can berecoveredessentially uncontaminated with virgin gas oil in the same boilingrange, for withdrawal from the cracking process or for recycling to thecracking unit after suitable treatment.

It is another object of the present invention to provide an.-improvedfractionating tower in which the desired separation of hydrocarbons maybe carried out.

ice

These and other objects of the present invention are accomplished byemploying a combination crude distillation and cracking process whichcomprises providing a fractionating column having an upper strippingsection, a lower stripping section, and a condensing section positionedbetween the said stripping sections. Crude oil is subjected todistillation to produce a reduced crude, which is then introduced intothe upper stripping zone of the fractionating column. The distillatefrom the distillation operation is subjected to a cracking operation andthe product of this treatment is introduced in, vapor form into thefractionating column at a point below the point of introduction of thereduced crude. These vapors have the eflfect of stripping the reducedcrude in the upper stripping section.

where it is further stripped with a vapor of higher temperature whichhas been introduced into the fractionating column at a point below thepoint of introduction of the said stripped product. This secondstripping operation produces a vapor containing cracked gas oil. Thevapor containing the cracked gas oil is passed to a condensing zone inwhich asubstantial amount of the cracked gas oil is condensed to aliquid. All of the condensed gas' oil is then withdrawn from thefractionating column, a portion of it being withdrawn from the processor recycled to the catalytic cracking stage and a further portion beingcooled and recycled to the said eondensin zone.

By providing a process of this nature, all of the condensed gas oil isremoved from the column without becoming mixed with the heavy bottomsfrom the reduced crude and from the heavy product of the cracking stage.Once these bottoms become mixed with the gas oil, separation isverydifficult, but such a problem is obviated by the use of the process ofthe present invention. The process also has the advantage of utilizinglarge volumes of hot process vapors for the purpose of stripping thereduced crude of gas oil with the effect that the volume of heavy fuelfinally produced may be kept at a minimum and lower than may beaccomplished in conventional operation involving atmosphericdistillation. A single heavy fuel fraction is obtained in which allheavy constituents uncontaminated by gasoil formed in the various stagesare combined, and which may, if necessary, be subected to a singlefiltering treatment to recover a final low sediment fuel oil of bestquality. Alternatively, this oil mayalso be fed to a visbreaking orcoking operation.

These advantages are in addition to the obvious saving of heat exchangeequipment resulting from the elimination of various intermediate heatingand cooling operations crude distillation unit to produce an overheadstream of light virgin naphtha, a separate heavy naphtha stream, a

still heavier stream of the kerosene and diesel oil range,

and light gas oil with a boiling range of about 430650- F., and reducedcrude bottoms. The reduced crude-bottoms at this stage may still containthe gas oil fraction intended in part at least for feed stock to a gasoil cracking step. The kerosene fraction may be recovered as productdirectly from the crude still, because further conversion will notsubstantially enhance its value as kerosene or diesel oil.

In certain cases, the process of the present invention may be simplifiedby eliminating the crude still and feeding the whole crude oil directlyinto the product fractionator. This may be possible if separate virginnaphtha and/or kerosene fractions are not required. When The strippedproduct is passed from the: upper stripping section to the lowerstripping section so operating, the whole crude may be fed to an upperportion of the lower stripping section of the product fractionator. Aheavy naphtha stream and gas oil cracking stock may be withdrawn fromthe fractionator and the latter subjected to various conversions,particularly catalytic cracking. The vaporized products of theseconversions may then be returned to the product fractionator at a pointbelow the crude feed point. Also, light naphtha and overhead gases fromthe fractionator may be reheated and returned to the fractionator toassist in crude stripping. Steam and other hot gases may also be addedat the bottom of the fractionator to aid in the stripping operation.

The invention may be best understood fromthe more detailed descriptionhereinafter, which refers to the accompanying drawings, in which:

Figure I is a schematic flow plan of a preferred modification of thecombination process in accordance with the. invention; and

Figure II is a view in section showing in detail the structure of theproduct fractionator wherein a series of 'fractionating trays orequivalent means is illustrated.

Referring now to the drawings, and particularly to Figure I, there areillustrated a crude still 10, a reforming unit 11, a catalytic crackingstage 12, and a product fractionator 13. The functions and co-action ofthese elements will be forthwith explained, using as an example therefining of a medium gravity crude of the type of Arabian Qatar crude ina refinery having a capacity of about 10,000 barrels of crude per day.It should be understood, however, that the system may be used for therefining of difierent types of crude on a larger or smaller scale in agenerally analogous manner.

In operation, the crude oil enters the crude still through line 14,usually at a temperature of about 750-800 F. and at a pressure of about40-70 p.s.i.g. The still 10 may be provided with a plurality ofhorizontal bubble cap plates to improve fractionation of the feed'in aconventional manner. For the purposes of the present invention, still 10may be operated to produce three distillate streams and distillationbottoms.

All crude constituents boiling below about 2500 F. are removed as avapor stream of light virgin naphtha overhead through line at atemperature of about 300- 350 F. This stream may amount to about 25% ofthe crude charged. A liquid stream of heavy naphtha having a boilingrange of about 250-500 F. is removed through line 16 from anotherportion of still 10. About 20-25% of the crude charged is recoveredthrough line 16. A kerosene or diesel oil out boiling within the rangeof about 500-700 F. and amounting to about 17-23% of the crude is drawn011 through line 17. The remainder of the charge, amounting to about30-50% and consisting predominantly of constituents boiling above600-700 F., is withdrawn as reduced crude through line 18 from thebottom of still 10. The kerosene cut removed through line 17 is normallysuitable for kerosene or diesel oil purposes without further treatment,and it may be passed directly to storage. The other fractions may betreated in. accordance with the present invention, as will besubsequently described. The light virgin naphtha vapors in line 15 maybe passed directly to a lower portion of product fractionator 13. Ifdesired, this vapor stream may be preheated to about 1000 1200 F. toconform with the heat requirements of fractionator 13. This may be doneby by-passing at least a portion of the vapors in line 15 through a coil19 located in connection with reforming furnace 11.

The heavy naphtha stream is passed through line 16 at a pressure ofabout 900-1100 p.s.i.g. to the thermal reformer 11. This reforming stagemay be of any conventional design well known in the art. As indicated inFigure I, it may comprise a conventional furnace 11 designed to providean oil residence time of about 18-25 liquidvolumes per volume ofreaction space per hour the fractionator 13 at a point substantiallyidentical withthe feed point of line 15. In Figure II of the drawingsthese two streams are shown as entering thebottom of Y fractionator 13at the same point through line 15.

The reduced crude in line 18 may be passed directly to the fractionator13, substantially at the temperature of its withdrawal from the still10. Line 18 feeds into the fractionator 13 at a point 18a above thecommon feed point of lines 15 and 20. In this manner the vapors suppliedthrough lines 15 and 20 pass upwardly'through'.

fractionator 13 against the downwardly flowing reduced crude to stripthe latter of vaporizable constituents.

This effect and the operation of fractionator 13 will be" described ingreater detail subsequently.

Gas oil hydrocarbons amounting to about 45% to 60% of reduced crude andboiling between about 600 F. and 1100 F. are withdrawn, preferably at atemperature of about 650-750 F., as a sidestream from the fractionator13 at a point above the reduced crude feed inlet 18a through line 22. Aportion of this withdrawn gas oil may be recycled through line 23 andheat exchanger 24- to a point higher or lower in the column in order toprovide reflux and temperature control. A" portion of this gas oil,however, passes through line 25 to the cracking stage 12. Anyconventional cracking" system adapted to convert gas oil rangehydrocarbons into lower boiling oils, particularly of the motorfu'elrange, may be used. While a thermal cracking operation may be used,it is generally preferred to provide'a catalytic process, which may beof a continuous or batch operation in fixed bed, moving bed, fluid orsuspensoid systems. supplied as preheat of process materials and/or assensible heat of exothermically regenerated catalyst, or' in' any otherconventional manner. Modified natural or synthetic clay or gel typecatalysts such as silica-alumina" composites or other conventionalcracking catalysts may be employed at temperatures of about 800-1000 F.and pressures of about atmospheric to 25 p.s.i.g., all in" a mannerknown to the prior art.

The total hydrocarbon efiluent from cracking stage 12 is passedsubstantially at the cracking temperature of about 900 F. through line26 to the fractionator'13 either at a point above the condensing section21, or below it, or both. In most cases the cracked material enters thefractionator 13 in the vaporous state to enhance the stripping action ofthe vapors supplied through lines 15' and 20.

" As indicated in Figures I and II, the fractionator 13 comprises anupper fractionating section 13a, an upper stripping section 27, a lowerstripping section 28, and an intermediate condensing section 21.Relatively low boiling constituents such as' gas and naphtha and heatingoil are taken off via the upper fractionating section 13a as shown.Reduced crude introduced near the upper part of the upper strippingsection is contacted'counter currently therein with rising hot vaporsfrom the cracking unit introduced at 26. The reduced crude descendingthrough the tower exchanges its virgin gas oil content with the crackedgas oil introduced through line- 26. By

the time the reduced crude reaches the cracked product feed point, allthe virgin gas oil has been stripped or replaced by cracked gas oil inthe same boiling range; The stripped reduced crude containing crackedconstituents in the gas oil boiling range is accumulated-on totaldraw-off tray 29 and flows around the'condensingsece- Heat required forcracking may be tion' 21 through line 30 and re-enters the column at thetop of the bottom stripping section 28. In this section the condensedportion of the cracked gas oil, together with the high boilingconstituents of the reduced crude introduced through line 18 and anyheavy constituents introduced from the cracking stage, is contacted withrising vaporsintroduced through line 15. These are the gaseous productsof the reformer 11 and the preheated light naphtha. taken from the topof crude still and reheated in coil 19, and also some catalytic crackingproductv vapors; This action strips the cracked gas oil from the heavybottoms, which are removed from the bottom of the fractionator throughline 31.

The. cracked gas oils stripped from the bottoms then pass upwardlythrough the total draw-off tray 32, which defines thebottom of thecondensing section 21. The draw-ofi trays 29 and 32 are so constructedas to allow vapors to pass upwardly through them, while preventingliquid from falling down to the next lower tray.

In the condensing section 21 a substantial percentage and preferably allof the cracked gas oil stripped from the liquid in stripping section 28is condensed on plates 33 and 34, while the uncondensed vapors passupwardly through the trap-out tray 29Iand are combined with crackingproduct vapors introduced via line 26 into the tower 13.' 'Although in.the preferred 'form of the invention substantially all of the crackedgas oil is condensed in the-condensing section '21, the. fractionatormay be operated in such a way that a portion of the cracked gas oilwillpass through the trap-out tray 29 in admixture with the reformerproduct and will exchange with the virgin gas oil entering with thereduced crude at line 18. The condensed gas oil is removed from thelower trap-out tray 32 through line 35, where it may either be recycledthrough condenser 36 and line 37 to return to condensing section 21, ormay pass through line 38 to subsequent or intermediate processing. Aportion of the condensed gas oil may be fed to another treating processsuch as a phenol treating plant and then recycled to the crackingsection. Suflicient cracked gas oil is recycled to the condensing zone21 in order to maintain the temperature therein in the range of about600-750 F.

A mixture of virgin and cracked gas oils is removed from fractionator 13through line 22 as previously described and recycled to the crackingstage.

By the provision of the condensing zone 21 a partial recycle of aso-called heart-cut gas oil fraction to the cracking zonemay beobtained. This permits closer control over the operating conditions ofthe cracking stage and improved cracking yields. In addition, the heavycrude bottoms removed through line 31 are uncontaminated by any gas oil.As pointed out, the system also provides a method for the selectivewithdrawal of a catalytically cracked gas oil which may be withdrawnentirely from the system and used for any desired purpose. Also, thiscracked gas oil can be subjected to a treating process to improve itscatalytic cracking qualities before return to the cracking section.

In some embodiments of the system it may be desirable to have a portionof the reformer products introduced into the fractionator 13 at thepoint where the cracked products normally enter, i.e., through line 26.Conversely, in some cases it might also be desirable to introduce aportion of the products of the cracking stage at the point where theeffluent from the reformer enters, i.e., through line in the bottomstripping zone 28 rather than at the bottom of upper stripping zone 27as shown by the drawing.

In a typical operation utilizing the present invention, 100 volumes ofreduced crude boiling above about 650 F. and containing about 48 volumesof gas oil boiling in the range of from 650-975 F. are fed to thefractionator at the point indicated on the drawings as 18a. The reducedcrude may be introduced at a temperature of about 700 F. Catalyticcracking reactor products'containing'72 volumes of recycle stock in the650975 F. gas oil boiling range are fed to the fractionator throughlines 26 and 15. Eighty percent of this material is introduced throughline 26, with the remaining 20% introduced at point 15, the latter inadmixture with thermal reformer product. The thermal reformer producthas a temperature of about 1050 F. so that the gas mixture enteringthe'bottom of the fractionator is 975 1000 F. The cracked materialenters through line 26 at a temperature of about 925 F.

If the condensing section 21 is not operated, the gas oil sidestream 25will consist of about volumes of material, of which 48 volumes will bevirgin material and 72 volumes cracked material, i.e., a 2/3 ratio ofvirgin cracked gas oil. However, when the condensing section 21 isoperatedby pumparound cooling through the cooler 36, a sidestream of 8volumes of gas oil in the 650-975 F. boiling range can bewithdrawnthrough the line 38. This sidestream will consist of 1-2volumes of virgin material and 7-6 volumes of cracked material, or aratio of virgin to cracked material of between 1/7 and l/ 3. Under theseconditions, sidestream 25 will consist of 112 volumes of material, ofwhich 47-46 volumes will be virgin material and 65-66 volumes will becracked material. Hence, in this operation the withdrawal of 7 volumes.ofcracked gas oil from the system results in the loss of only 1 2volumes of virgin gas oil, which is a valuable catalytic cracking feedstock. Without the operation of. the condensing section, 4-7 volumes ofvirgin material would have to have been withdrawn with the 7 volumes ofcracked gas oil through sidestream 25.

' During such an operation the temperature at the top of the condensingsection 21 is maintained at 700 F., while at the bottom a temperature of800 F. is maintained. Condensed gas oil withdrawn at 35 has atemperature of about 750 F., and the portion recycled through cooler 37and reintroduced through pipe 38 has a temperature of about 550 F.Relatively high boiling material, i.e., material boiling above about 975F., is withdrawn from the bottom of the fractionator at 31 at atemperature of about 825 F. Relatively low pressure in the order of 8p.s.i.g. is maintained in the bottom stripping section 28.

While there has been described herein a specific embodiment of thepresent invention, together with certain equivalent processes, otherequivalent steps and processes will occur to those skilled in the priorart. The process of the present invention is intended to cover such arange of equivalents, and is not to be limited to the specificembodiments herein described.

- What is claimed is:

1. In a combination fractionating and cracking process, the improvementwhich comprises providing a first upper stripping zone, a second lowerstripping zone, and an intermediate condensing zone, introducing crudeoil con-.

taining virgin constituents in the gas oil boiling range into I saidfirst upper stripping zone, introducing the product of a hydrocarboncracking treatment in vapor form containing cracked constituents in thegas oil boiling range into said first stripping zone, at a point belowthe point of introduction of said crude oil containing virginconstituents, stripping said crude oil with said vapors in said firststripping zone to remove most of the said virgin gas oil constituentsfrom said crude oil, passing the stripped product containing somecondensed cracked gas oil constituents to said second stripping zone andbypassing said intermediate condensing zone, further stripping saidstripped product in said second lower stripping zone with a vapor ofhigher temperature to produce a vapor containing cracked gas oilconstituents, passing the cracked gas oil-containing vapor to saidintermediate condensing zone, condensing said cracked gas oilconstituents in said condensing zone, withdrawing all of the condensedcracked gas oil from said condensing zone, recycling a portion ofsaidcondensed gas oil to said condensing zone,

using the unconde'nsed vapors from the condensing' zone" as additionalstripping vapors in said first stripping :zone

andwithdrawinga sidestream' from the fractionator at the top of saidfirst shipping zone comprising a mixture of cracked and virgin gas oilconstitutents and bypassing at least a portion of said stream to acracking zone to produce said product of a hyrocarbon crackingtreatment.

2. The process according to claim 1 wherein said vapor of highertemperature introduced into said lower stripping section is the productof a hydrocarbon reforming treat-' ment.

3. The process according to claim 1 wherein said reduced crude in saidfirst stripping zone isstripped by a mixture of said, product of saidcracking treatment and said vapor of higher temperature.

4 The process according to claim f1 wherein the stripped product in saidsecond stripping section is further stripped by a mixture of saidproduct of said cracking treatment and said vapor of higher temperature.

SLThe process according to claim 1 whereinthe stripping vapors in eachof said stripping sections comprise a mixture of said product of saidcracking treatment and said vapor of higher temperature.

6. In a combination fractionating and cracking process, the improvementwhich comprises providing a fractionating column having an upperstripping section, a lower' stripping section, and a condensing: sectionpositioned between said stripping sections, subjecting crude oil'to'form into said fractionating column at a point below the point ofintroduction of said reduced crude, stripping saidreduced crude withsaid vapors in said upper stripping section, passing the strippedproduct to said lower stripping section while bypassing said condensingsection, further stripping said product in said lower stripping sectionwith" the product of said reforming treatment in vapor form,- introducedinto said lower stripping section at a point below the point ofintroduction of said stripped product, to produce a vapor containingcracked gas oil, passing the gas oil-containing vapor to said condensingzone, condensing said cracked gas oil in said condensing zone,- passingvapor containing uncondensed gas oil from said condensing zone to saidupper stripping section, withdrawing all of the condensed gas oil fromsaid condensing.-

zone, recycling a portion of said condensed gas oilto said crackingtreatment, recycling a further portion of said condensed gas oil to saidcondensing zone, and

withdrawing said uncondensed gas oil from said upper stripping section.

7. The process according to claim 6 wherein said product of the crackingtreatment is introduced at'a" temperature of from about 800 F. to about1000 F. and

the product from the reforming treatment is introduced at a temperatureof from about 1000 F. to about 1200' F.

8. The process according to claim 6wherein a portion of the condensedgas oil withdrawn from said condensingzone of said fra'ctionating columnis subjected to an additional treating step prior to recycling'to saidcrackin'g' treatment.

9. The process according to claim 6 wherein a portion of the condensedgas oil withdrawn from said con-' densing zone of said fractionatingcolumn is subjected to. phenol treatment prior to recycling to saidcracking treat-' ment.

References Cited in the tile of this patent UNITED STATES PATENTS2,018,309 Jones Oct. 22, 1935- 2,082,224 Snow June 1, 1937 2,130,596Ocon Sept. 20, 1938 2,644,785 Harding et al. July 7, 1953' 2,777,801Bittner et a1. Ian.- 1, 1957

1. IN A COMBINATION FRACTIONATING AND CRACKING PROCESS, THE IMPROVEMENTWHICH COMPRISES PROVIDING A FIRST UPPER STRIPPING ZONE, A SECOND LOWERSTRIPPING ZONE, AND AN INTERMEDIATE CONDENSING ZONE, INTRODUCING CRUDEOIL CONTAINING VIRGIN CONSTITUENTS IN THE GAS OIL BOILING RANGE INTOSAID FIRST UPPER STRIPPING ZONE, INTRODUCING THE PRODUCT OF AHYDROCARBON CRACKING TREATMENT IN VAPOR FORM CONTAINING CRACKEDCONSTITUENTS IN THE GAS OIL BOILING RANGE INTO SAID FIRST STRIPPINGZONE, AT A POINT BELOW THE POINT OF INTRODUCTION OF SAID CRUDE OILCONTAINING VIRGIN CONSITTUENTS, STRIPPING SAID CRUDE OIL WITH SAIDVAPORS IN SAID FIRST STRIPPING ZONE TO REMOVE MOST OF THE SAID VIRGINGAS OIL CONSTITUENTS FROM SAID CRUDE OIL, PASSING THE STRIPPED PRODUCTCONTAINING SOME CONDENSED CRACKED GAS OIL CONSTITUENTS TO SAID SECONDSTRIPPING ZONE AND BYPASSING SAID INTERMEDIATE CONDENSING ZONE, FURTHERSTRIPPING SAID STRIPPED PRODUCT IN SAID SECOND LOWER STRIPPING ZONE WITHA VAPOR OF HIGHER TEMPERATURE TO PRODUCE A VAPOR CONTAINING CRACKED GASOIL CONSTITUENTS, PASSING THE CRACKED GAS OIL-CONTAINING VAPOR TO SAIDINTERMEDIATE CONDENSING ZONE, CONDENSING SAID CRACKED GAS OILCONSTITUENTS IN SAID CONDENSING ZONE, WITHDRAWING ALL OF THE CONDENSEDCRACKED GAS OIL FROM SAID CONDENSING ZONE, RECYCLING A PORTION OF SAIDCONDENSED GAS OIL TO SAID CONDENSING ZONE, USING THE UNCONDENSED VAPORSFROM THE CONDENSING ZONE AS ADDITIONAL STRIPPING VPORS IN SAID FIRSTSTRIPPING ZONE AND WITHDRAWING A SIDESTREAM FROM THE FRACTIONATOR AT THETOP OF SAID FIRST STRIPPING ZONE COMPRISING A MIXTURE OF CRACKED ANDVIRGIN GAS OIL CONSTITUTENTS AND BYPASSING AT LEAST A PORTION OF SAIDSTREAM TO A CRACKING ZONE TO PRODUCE SAID PRODUCT OF A HYDROCARBONCRACKING TREATMENT.